1. Field of the Invention
The present invention relates to a toner for developing an electrostatic latent image.
2. Description of the Related Art
As a conventional fixing system of toner, contact heating-fixing systems such as a heat roll-fixing system has been employed widely. In a fixing apparatus used in the heat roll-fixing system, a heat roll and a press roll are provided and a recording sheet supporting toner images is allowed to pass through a contact section (a nip section) between the heat roll and press roll so that the toner images are melted and fixed on the recording sheet.
In such contact heating-fixing systems represented by the heat roll-fixing system, the toner images on the recording sheet contact with the surface of a heating member (e.g. the heat roll) in a contact heating-fixing apparatus in order to be fixed. Therefore, it is necessary to prevent an offset phenomenon in which a part of the toner images is stuck to the heating member and the part is transferred on the next recording sheet to soil it.
The technique for preventing the offset phenomenon, in which a fixing oil such as silicone oil and the like is coated on or impregnated into the heat roll or the press roll of the fixing apparatus, has been known. However, in a monochrome image-forming apparatus, an oil-less fixing apparatus in which a fixing oil-coating system is omitted or the amount of the fixing oil to be coated is decreased is used from the viewpoint of miniaturization and low-cost of the fixing apparatus. When such a oil-less fixing apparatus is used, a wax is added in a toner as an anti-offset agent.
Moreover, the technique has been also known, in which not only the general wax for preventing the offset but also the different kind of wax are added in order to accomplish high improvements of toner-functions, such as a prevention of stains and blurs (smear) occurring on an image because of a rub at the time of both-sides copying or an improvement of low-temperature fixabilities and the like. In order to highly improve anti-offset properties and the other functions as just described, it is efficient to increase the content of various kinds of waxes in toner particles.
On the other side, in an image forming apparatus in which a full-color image is formed by using plural-color toners, the fixing oil such as silicone oil and the like has been conventionally coated on or impregnated into the heat roll or the press roll and, thereby, the prevention of the offset phenomenon and the maintenance of separabilities of a recording sheet from the heat roll and/or the press roll have been performed. The oil-less fixing of the full-color toners used in such a full-color image-forming apparatus has also been examined. In a general toner for reproducing a monochrome image, a resin having a high viscoelasticity can be used as a binder resin. Therefore, an intermolecular cohesive force is high at the time of toner-melting (being fixed) and, even when the content of wax is low, separabilities from fixing rollers and anti-offset properties can be maintained. However, when the binder resin having a high viscoelasticity is used in a full-color toner, light transmission properties, gloss properties and reproducibility of color become insufficient. Accordingly, in the full-color toner, the degree of dependence on wax for anti-offset properties and separabilities is high and it is necessary to increase the content of wax.
However, in general, a wax is incompatible with a binder resin and, when the blending amount of wax in raw material is increased in order to increase the content of wax, the amount of wax liberating from a toner particle is increased in the step of grinding a kneaded product at the time of producing toner. Therefore, the amount of wax that is actually comprised in a toner particle becomes lower than that in mixing raw material and, as a result, it is impossible to make wax be effectively comprised in a toner particle. Accordingly, the effects of improvements for toner properties (anti-offset properties, separabilities between a fixing member (a heat member and/or a press member) and a recording sheet, low-temperature fixabilities, smear-preventive properties) can not be shown sufficiently by adding wax under the present conditions. Moreover, when the liberating wax is increased, problems in which a maintenance of heat resistance of toner is deteriorated or filmings occur on an image arise.
Therefore, for the purpose of preventing the occurrence of the liberating wax, a trial in which wax is dispersed in a comparatively small particle size in order to restrict an exposure of wax to the surface of a toner particle has been carried out. However, the effects of improvements for the above-mentioned toner properties can not be shown sufficiently by adding wax.